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Old 05-05-02, 02:36 PM   #1
fietser_ivana
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Calibrating geeks & bike computers

Guyz and gals, I've got a question for you.. but here's the story.

Friday night, I finally fitted a new bike computer to my new bike. Saturday would be the second time I'd take out my brand new bike for a bike ride and while I'd bought the computer before the 1st ride, it couldn't be fitted for lack of a special computer stick.

Normally I always measure up the circumference by taking chalk & a measuring tape to a parking lot or any other empty terrain with straigth white lines or anything else that's linear.
Roll out the bike a couple of times, measure distance, add the distances and divide by of measurements to get the average.

But as it was late at night already, I had to make do with the suggested circumference of Cateye, which I know to be faulty since the socalled 25mm listed (1 inch) was in reality just 20mm wide (yes, I have calipers).

The next day I'd be riding with someone I'd never met before. Guess what my date had done? The computer had been installed by the LBS and in the 3 years he'd been cycling with it, he had never thought of calibrating it... aaarrrrrgggghhhh!!!!

Sorry!!! I was even mad at myself for being too lazy to install the computer so late that I hadn't had time for measuring the circumference (could have done that sans computer as well) and here there's someone who didn't care at all.

OK, a cycling buddy of mine is pretty eccentric, so it wasn't much of a surprise to me that he had asked a LBS to install a bike computer and told him to err on the safe side. But someone who claims to have an administration job & used to be in techique????

Oh well, it's not all that important, it just really took me by surprise as measuring is so simple and I assumed that everyone would be like me and prefer a well-calibrated computer...

So what about you? Am i the only calibration geek here, or are there others like me? Are you even aware of how to do it? Do you care??

Curious calibration geek Ivana
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Old 05-05-02, 03:31 PM   #2
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I had never calibrated my computer because it seemed closeThis year I began commuting and wanted an accurate distance. The directions were long gone, so I looked at Sheldon Brown's instructions for my model. I chalked a tire, and measured.

Time for the riding test. My community has a trail that extends south to another community. They have marked the miles on the trail. I zeroed out my trip at the zero mile marker.

When I hit the 1 mile point, my computer read 1.15 miles. Just to make sure, I headed to the next marker.

At the 2 mile marker, my computer read 2.30 miles. So, I am exactly 0.15 miles off. I will just have to recalibrate and find the darn directions. I headed for the next marker.

At the 3 mile marker, my computer read 3.30 miles. For all of you following along and expecting the computer to read 3.45, I was stumped. I continued to the 4 mile marker and it read 4.30. The five mile marker read 5.30.

The 3, 4, & 5 mile markers are in the other community. Apparently, the 1 & 2 mile markers are off by 0.15 miles in my community.

My computer is accurate.
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Old 05-05-02, 03:48 PM   #3
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I calibrate for all my bikes that have a computer. AND, believe it or not, I don't calibrate for my bikes that don't have a computer. For those, I don't consider it necessary.
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Old 05-05-02, 03:55 PM   #4
fietser_ivana
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I'd never trust kilometer markers to be accurate, so I'd never think of calibrating a computer by them.. heck I don't even trust the 100 meter markers... I just roll out the bike 5 times.

What's so difficult about that? It just requires a chalk or any other marker, and a tape of exactly 1 meter length and preferably longer...

Ah, that's what it is ... you're into imperial ;-) but both people I was commenting on were using the metric system as well..

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Old 05-05-02, 03:58 PM   #5
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There is a section of bike path with very accurate mile markers, and I have calibrated (and adjusted) my computer to to be pretty accurate - but erring on the conversative side - i.e., I my computer shows about about .01 mile less per mile then the actualt distance. Just couldn't seem to resolve that last .01 mile, it was either a bit too much or a bit too little!! I chose the bit too little!!
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Old 05-05-02, 04:29 PM   #6
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Calibration and milage geeks unite! I am so careful to calibrate, that i re-calibrate when i change the tire treads. Believe it or not, the tread and even the inflation (95psi vs. 110psi) can make a difference for me. I even think that it makes a difference when you measure by marking a revolution under load vs. with no one in the saddle. This theoretically should simulate a ride.
I figure by marking a complete revolution of the rear tire. I don't want any cheap miles or even cheat myself.
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Old 05-05-02, 06:09 PM   #7
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Since most of my rides are on country roads, I have a couple measured out so that I can check my computer. So far it's even with my trucks odometer.
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Old 05-05-02, 06:26 PM   #8
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Do you have a 400m athletics track nearby?
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Old 05-06-02, 10:53 AM   #9
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After I got my 'Bent, I decided to get a computer, for me and my daughter. I found (by using the default on both) they didn't match. Of course she had 20" MTB tires and I had a 20" street tire. So after asking here how to get them in sync. I did the measure thing. Found her's was just 2 MM off the default on the measuring (left hers alone) but mine was like 3 1/4" off. Calibrated mine and now they are so close can't tell the differance. And the hardest part was just reading the fine print in the instructions. Getting old is hell. I also found out the almost 7 mile loop we were riding was really just over 5 miles. Dang, now I have to ride even more honey, I swear
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Old 05-07-02, 12:21 PM   #10
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I routinely forget to "turn on" my Cateye cordless computer when I get on my XC bike. I don't lose too much sleep over it.
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Old 05-07-02, 04:02 PM   #11
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Here's the OP again.. I've cycled for 3 months without a computer and got over it after a while.
The pro was that I was no longer concerned with speed
The con was, that it was harder to guess distances.
However, after a long time of touring I learnt that I average about 10 km/hr on the flats including breaks.

But I fitted another computer once back in California. I did wait about a week or longer before refitting it after having bought the computer... as I wanted it to be as exact as possible.

Ivana
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Old 05-07-02, 04:35 PM   #12
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I just went with the numbers that mached my tire size in the instructions.I have set a computor by measuring the distance the valve stem traveled in 1 revolution.Trying to keep the front wheel rolling in a straight line ain't for me.I don't think I ever rode with a group and our mileage matched.Some varied by miles on a 54 miler the other day.I like to see the distance,rpm and average speed,but try not to get to hung up about them.
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Old 05-07-02, 04:45 PM   #13
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It seems that a lot of people just don't get it..

While I was able to adapt again to not having a computer, heck I never had one on my shopping bike... I want it to be ACCURATE when I DO use one.. I take pride in calibrating it as exactly as possible. It's just that.. a mathematician's/engineer's mind set... nothing more nothing less. it has nothing to do with getting hung up about speed or distance..

OK I admit.. I used to record the exact distance travelled (often still do) as well as time spent cycling & total time.. this is because I was (and still am) a randonneur, who has to deal with time limits.. you get 20 hours to complete 300 K, 40 hrs to complete 600K, so this involves a lot of calculating about the necessary speed for a given part of the course. A lot of the itineraries we have also say "0.5 K straight on.. I want to be certain that the distance is correct..

It's both a matter of what sport you play and educational background.. I just thought everyone had the same ideas as I had and rolled out their bike.. apparently not!
BTW, you don't need to cycle the bike, just straddle it and wheel the bike over a white line for 1 circumference while pushing down at the bars. The reason I measure 3-5 times is that there's a certain error in the measurements.

Ivana
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Old 05-07-02, 05:15 PM   #14
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I got what you was saying.You asked what we did.I just think the numbers in the instructions are close enough.If you factor in that riding in different parts of the road is going to give different distances.If you have 2 people with perfectly matched computors riding side by side down the same twisty turning road,their mileage is going to be different just because their lines are different.

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Old 05-07-02, 07:10 PM   #15
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Here's something to consider. As you ride the weight of the rider deforms the tire at the contact patch. How do you compensate for this? If you're just measuring the diameter by running the tire out unweighted are you getting an accurate measurement?

Oh, I don't use a computer. I enjoy the qualitative aspects of cycling more than the quantative ones.
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Old 05-08-02, 01:35 AM   #16
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Hillyman was right... yes, there is a slight difference because we all ride slightly different courses, eg I tend to zigzag more than others when balancing the rather large bars. When I travelled with a group through Czech & Slovak Republic, I'd call myself the small zigzagger while another guy, Dan from Sweden was the big zigzagger.. no wonder, as he was able to climb nearly every hill with his single-speed bike, but only when he used half of the road to get there...

There is not really that much of a difference in circumference of a weighted bike and an unweighted bike, but to get it right, I straddle the bike and press down the handlebars heavily so as to 'mimick' a loaded bike.. Since you're mostly riding without luggage, there won't be a lot of pressure on the front tube where the computer sensor is mounted...
See, everything is solved.. I may be more obsessive than others about getting things right, but there's only so much you can do..

Of course I could try hanging full & heavy pannirers on the bike, but 80% of the rides I take with that bike are done unloaded. And these are the rides where it matters to have a correct distance if you want to be able to follow instructions..

Ivana, still amazed at a difference of 10 kilometers (my total 122 K, his 132 K) between 2 riders who rode the very same distance...ok, it was only 9 since he came to my place first..
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Old 05-08-02, 03:38 AM   #17
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I simply used the guide size from the manual, and roughly checked it against old milestones.

Do you re-calibrate to take account of tyre wear

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Old 05-08-02, 04:02 AM   #18
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No, of course not.. but I do recalibrate when buying a new tyre, especially when it's a different brand as tyre sizes vary enormously from one brand to another.

Ivana, owning a 20mm tyre that is advertized to be 25mm.
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Old 05-08-02, 04:23 AM   #19
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Count me in, I'm a calibratin' geek. But not to the extent that I would re-calibrate every two weeks or so. I check it when I change tyres.

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